A prior art temperature detection circuit incorporated in a semiconductor chip is as shown in FIG. 5A. One end of current sources 50 and 60 is coupled to a power supply VCC. The other end of the current source 50 is coupled to one end of a zener diode D1 having a positive temperature coefficient. The other end of the zener diode D1 is coupled to a ground potential. The zener diode D1 is coupled in a reverse direction between the power supply 50 and ground potential. The voltage at the other end of the current source 50 is divided by resistors R1 and R2. The resistors R1 and R2 are coupled to a first detection node Vx. The other end of the current source 60 is coupled through a second detection node VD to one end of a diode D2 having a negative temperature coefficient. The other end of the diode D2 is connected to a ground potential. The diode D2 is coupled in a forward direction between the current source 60 and ground potential. An input of a comparator that outputs a detection signal is coupled to the first detection node Vx and second detection node VD.
This circuit operates as follows. First, under a normal condition, the semiconductor chip having the temperature detection circuit is within a usage temperature range below its maximum usable temperature TMAX (FIG. 5B). Reference numerals 52 and 62 in FIG. 5B denote the temperature dependency of the voltage at the first and second detection nodes Vx and VD, respectively. This circuit is designed so that the voltage at the second detection node VD is higher than that of the first detection node Vx under a normal operating condition. Thus, the output of the comparator 70 is at a high level. If the zener diode D1 having a positive temperature coefficient is used, the voltage at the first detection node Vx increases as the temperature of the semiconductor chip rises (52). On the other hand, because the diode D2 has a negative temperature coefficient, the voltage at the second detection node VD falls as the temperature of the semiconductor chip rises (62). When a sense (detection) temperature TD is reached where the voltages at the first detection node Vx and second detection node VD are equal, the output of the comparator 70 goes low, thereby detecting that a high temperature region exceeding a safely usable temperature has reached. Thereafter, an action, such as cooling the semiconductor chip in order to protect it against overheating, is taken as needed.
In this way, the sense temperature TD defines the temperature limit that permits the semiconductor chip and so forth to be used safely. As the sense temperature TD becomes higher, the usable temperature range also becomes wider accordingly; thus, a higher sense temperature TD is desirable for the user of the semiconductor chip and so forth. Because the temperature cannot be increased above the maximum usable temperature (breakdown temperature) TMAX, the sense temperature TD should desirably be smaller than the maximum usable temperature TMAX but as close as possible thereto.
However, when the temperature detection circuit or semiconductor chip of this type is produced in volume, there is variability due to variations in the manufacturing process, so that the sense temperature TD varies relative to its target value within a range having a certain margin. Thus, it is necessary to lower (move away) the target value sufficiently from the maximum usable temperature so that the maximum usable temperature TMAX is not exceeded even when the sense temperature TD varies. When the target value is lowered, the upper limit of the user-operable temperature is also lowered. As a result, it is difficult to meet the demand for devices and their overheat protection circuits that operate in a high temperature region, such as printer heads of inkjet printers.
In the circuit example described above, significant variations in the sense temperature occur due to variations in the zener voltage of the zener diode D1 and in the on-voltage of the diode D2. The present invention is intended to minimize the influence of variations in the manufacturing process and so forth and to provide a temperature detection circuit that can be operated in a high temperature range near its maximum usable temperature TMAX.
For use in a high temperature range near the maximum usable temperature, it is necessary to detect the sense temperature with greater accuracy than the prior art. This is because if the accuracy is poor, the sense temperature TD cannot be brought close to the maximum usable temperature TMAX. It is an objective of the present invention to provide a temperature detection circuit that detects the temperature with high accuracy.
In order to assure operating safety of the semiconductor chip having the temperature detection circuit, it should desirably have a hysteresis characteristic. It is an objective of the present invention to provide a temperature detection circuit that can readily incorporate a hysteresis characteristic.
Furthermore, because the zener voltage is typically above 5 volts, a power supply having a high voltage above 5 volts is required for the circuit shown in FIG. 5A. Thus, it is difficult to operate the circuit with a low voltage. It is an objective of the present invention to provide a temperature detection circuit that operates at a low voltage.